With the recent proliferation of microcomputers and colour monitors, there has been a massive growth in the amount of information available for display in colour. Presentation of such information has created a demand for hard copy, for example on paper sheets, on opaque plastics films (which can be more robust than paper sheets) and also on transparent sheets, which are suitable for viewing in transmission mode, e.g., by overhead projection.
Ink jet printing is well established as a technique for printing such information including multi-colour graphics. In ink jet printing, an ink droplet is projected on to an ink receptive sheet at high velocity (e.g., up to 20 m/s). Movement of the ink jet may be computer controlled, and characters may be formed and printed rapidly. To derive advantage from this high speed operating capability requires an ink receptive sheet, which will rapidly absorb the high velocity ink droplet without blotting or bleeding.
To improve image resolution, ink jet printers have been developed which are capable of providing a greater density of ink droplets, for example up to about 1440 dots per inch (dpi). For a given droplet size, the increased `dpi` of such printers as compared with that of lower resolution printers has the effect of increasing the volume per unit area of ink to be absorbed.
Generally, an inkable sheet suitable for use with ink jet printers comprises a substrate carrying an ink absorbent layer. For use in overhead projection, the substrate must, of course, be transparent and transparent polyethylene terephthalate or transparent polyvinyl chloride films are commonly used. In the case of opaque plastics films, opaque polyethylene terephthalate or opaque polyvinyl chloride films are commonly used. The ink absorbent layer typically comprises a polymer or a mixture of polymers and combinations of cellulosic polymers such as nitrocellulose, carboxymethyl cellulose and especially hydroxyethyl cellulose; gelatins; vinyl polymers such as polyvinyl acetate and polyvinyl pyrrolidone; and acrylic polymers such as polyacrylic acid are described in EP-A-0156532, EP-A-0232040 and EP-A-023 3703.
A further consideration is that the use of aqueous based inks having a high water content (possibly up to 95%) is becoming more common. Such inks used for multicolour printing (i.e. cyan, magenta, yellow and black) generally consist of an aqueous solution of an appropriate dye. However, recently there has been a trend to the use of black ink consisting of an aqueous dispersion of a pigment together with a polymer whose function is to hold the pigment together when the ink has dried. Under certain circumstances, the presence of such a polymer in the ink can cause problems, in that if the ink polymer and the polymer in the absorbing layer of an inkable sheet become intermixed, swelling and subsequent shrinking of the ink dot can result, leading to cracking of the dried dot with a consequent reduction of the optical density. This is particularly serious in the transmission mode, where the optical density can be reduced by up to 50% due to light passing through the cracks.
The present invention aims to alleviate the above problems associated with the prior art.